Torque sensor

ABSTRACT

A torque sensor ( 50 ) detects an input torque input into a torsion bar ( 51 ) using a magnetic force generating part ( 60 ), a rotating magnetic circuit ( 69 ), a fixed magnetic circuit ( 90 ), and a magnetic sensor ( 98 ). The rotating magnetic circuit ( 69 ) comprises a first soft magnetic member ( 70 ) and a second soft magnetic member ( 80 ), each of which comprises a magnetic ring ( 73, 83 ), a magnetic tip ( 71, 81 ) facing the magnetic force generating part ( 60 ), and a magnetic column ( 72, 82 ) connecting the tip ( 71, 81 ) and the ring ( 73, 83 ). By disposing the first soft magnetic member ( 70 ) and the second soft magnetic ring ( 80 ) so as to face each other, the soft magnetic rings ( 70, 80 ) are formed in an identical shape that can be manufactured by press-working a plate material, thereby realizing a compact and lightweight torque sensor ( 50 ).

FIELD OF THE INVENTION

This invention relates to a torque sensor for detecting a torque actingon a rotating body without contact.

BACKGROUND OF THE INVENTION

JP 2007-240496 A published by the Japan Patent Office in 2007 disclosesa torque sensor for a steering system of a vehicle. The torque sensordetects a steering torque input into a torsion bar without contact.

The torque sensor comprises a magnetic force generating part whichrotates together with a base of the torsion bar that has been operatedto rotate, a rotating magnetic circuit which rotates together with anoutput shaft connected to a tip of the torsion bar, a fixed magneticcircuit fixed to a housing, and a magnetic sensor which detects amagnetic flux density in the fixed magnetic circuit.

As the torsion bar undergoes a torsional deformation due to an inputtorque, the magnetic force generating part rotates relative to therotating magnetic circuit, and the magnetic flux density which therotating magnetic circuit receives from the magnetic force generatingpart varies.

The rotating magnetic circuit and the fixed magnetic circuit areconfigured to transmit magnetic flux without contacting. By detectingthe density of the magnetic flux in the fixed magnetic circuit using themagnetic sensor, it is possible to detect a torque acting on the torsionbar.

To transmit the magnetic flux from the rotating magnetic circuit to thefixed magnetic circuit, the rotating magnetic circuit comprises a pairof soft magnetic rings. The fixed magnetic circuit comprises a pair ofmagnetic collecting rings fixed to the inner peripheral surface of thehousing.

One of the magnetic collecting rings is disposed to permanently face theouter peripheral part of one of the soft magnetic rings. The othermagnetic collecting ring is disposed to permanently face the outerperipheral part of the other soft magnetic ring. According to thisarrangement, when the magnetic collecting ring rotates relative to thecorresponding soft magnetic ring, the magnetic flux density transmittedfrom the soft magnetic ring to the magnetic collecting ring does notvary. By providing such a magnetic flux transmission mechanism, themagnetic sensor can detect the torque acting on the torsion bar withoutcontact.

SUMMARY OF THE INVENTION

In this torque sensor, however, it is difficult to manufacture the softmagnetic rings by press-working a plate material due to the complicatedshape thereof. An expensive manufacturing process such as metal castingor sintering is required to form the soft magnetic rings. It is alsoinevitable that the torque sensor becomes large in size, especially inthe axial direction, because of the complicated shape of the softmagnetic rings.

It is therefore an object of this invention to form the soft magneticrings into a simple shape so that they can be manufactured bypress-working a plate material, thereby realizing a compact andlightweight torque sensor.

To achieve the above object, this invention provides a torque sensorcomprising a housing, a torsion bar inserted into the housing, amagnetic force generating part which rotates together with an end of thetorsion bar while generating a magnetic flux in a direction of arotation axis of the torsion bar, and a rotating magnetic circuit whichrotates together with another end of the torsion bar.

The rotating magnetic circuit comprises a first soft magnetic member anda second soft magnetic member.

The first soft magnetic member comprises a first magnetic ring disposedaround the torsion bar, first magnetic tips disposed in a peripheraldirection at equal angular intervals so as to face the magnetic forcegenerating part in the direction of the rotation axis, and firstmagnetic columns connecting the first magnetic tips magnetically to thefirst magnetic ring.

The second soft magnetic member comprises a second magnetic ringdisposed around the torsion bar in an offset position from the firstmagnetic ring in the direction of the rotation axis, second magnetictips disposed in a peripheral direction at equal angular intervals suchthat the second magnetic tips and the first magnetic tips are disposedalternately, and second magnetic columns connecting the second magnetictips magnetically to the second magnetic ring.

The torque sensor also comprises a fixed magnetic circuit fixed to thehousing to surround the rotating magnetic circuit so as to accept amagnetic flux from the rotating magnetic circuit without contact, and amagnetic sensor which detects a magnetic flux density in the fixedmagnetic circuit.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a power steering devicecomprising a torque sensor according to this invention.

FIG. 2 is an exploded perspective view of essential parts of the powersteering device including the torque sensor.

FIGS. 3A and 3B are perspective views of a rotating magnetic circuitaccording to this invention in a broken-down state and a built-up state.

FIG. 4 is a plan view of essential parts of the torque sensorillustrating the positional relationship between a magnetic forcegenerating part and the rotating magnetic circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a power steering device 1 for avehicle transmits an input torque that is input into an input shaft 10from a steering wheel of the vehicle to an output shaft 20 via a torsionbar 51. The rotational torque of the output shaft 20 is then transmittedto a steered wheel via a rack-and-pinion mechanism.

The power steering device 1 comprises an assisting mechanism using anelectric motor to assist a steering operation. The assisting mechanismcomprises a worm wheel 9 fixed onto the outer periphery of the outputshaft 20 and a worm 19 engaging with the worm wheel 9. The electricmotor drives the worm 19 to rotate such that an auxiliary torque isinput into the output shaft 20 in a direction of the steering operation.

To cause the assisting mechanism to generate the auxiliary torquecorresponding to the input torque input into the input shaft 10 when thesteering wheel is operated, the power steering device 1 comprises atorque sensor 50 which detects the input torque input into the inputshaft 10. By controlling a power current driving the electric motor onthe basis of a detected torque detected by the torque sensor 50, anauxiliary torque proportional to the input torque can be exerted on theoutput shaft.

The input shaft 10 is supported by the housing 30 via a ball bearing 7so as to be free to rotate. The input shaft 10 is formed to have anaxial hollow portion. The torsion bar 51 is accommodated in the axialhollow portion of the input shaft 10. A base of the torsion bar 51 isfixed to the input shaft 10 using a pin 5 while a tip of the torsion bar51 engages with the output shaft 20 via a serration 4.

A dust seal 6 is interposed between the housing 30 and the input shaft10. The output shaft 20 is supported by the housing 30 via a ballbearing 8 so as to be free to rotate. An outer periphery of a lower endof the input shaft 10 is supported by the output shaft 20 via slidebearing 3.

In the structure described above, the input shaft 10 and the outputshaft 20 can rotate relatively about an identical rotation axis within atorsional deformation range of the torsion bar 51.

Referring to FIG. 2, the torque sensor 50 comprises a magnetic forcegenerating part 60 which rotates together with the input shaft 10, arotating magnetic circuit 69 which rotates together with the outputshaft 20, a fixed magnetic circuit 90 fixed to the housing 30, and amagnetic sensor 98 which detects a magnetic flux density in the fixedmagnetic circuit 90.

The magnetic force generating part 60 comprises a magnet ring 63 fixedonto the input shaft 10 via a back yoke 61.

Referring to FIG. 4, the magnet ring 63 is formed by six arc-shapedmagnets made from a hard magnetic material. Each of the arc-shapedmagnets has a tip magnetized as an N-pole and another tip magnetized asan S-pole. The arc-shaped magnets are arranged such that the N-pole ofon magnet and the S-pole of an adjacent magnet abut each other.According to this construction, the magnet ring 63 has six N-poles andsix S-poles which are disposed alternately along a circle at equalangular intervals.

Referring again to FIG. 2, the back yoke 61 is a cylindrical memberformed from a soft magnetic material and press-fitted onto the outerperiphery of the input shaft 10. The magnet ring 63 is fixed in advanceonto the outer periphery of the lower end of the back yoke 61.

The back yoke 61 functions as a fixing member that fixes the magnet ring63 onto the input shaft 10 as well as a yoke that transmits magneticflux between an N-pole and an S-pole which abut on each other. Bycausing the back yoke 61 to contact the upper surface of the magnet ring63, the magnetic flux of the magnet ring 63 is concentrated on the lowersurface of the magnet ring 63.

It is also possible to provide a fixing member to fix the magnet ring 63onto the input shaft 10 separately from the back yoke, whereby the backyoke is interposed between the fixing member and the magnet ring 63.

The rotating magnetic circuit 69 comprises a first soft magnetic member70 and a second soft magnetic member 80 for receiving the magnetic fluxgenerated by the magnet ring 63, a fixing member 77 fixed onto theoutput shaft 20, and a resin-molded body 87 which fixes the first softmagnetic member 70 and the second soft magnetic member 80 onto thefitting member 77.

Referring to FIGS. 3A and 3B, the first soft magnetic member 70comprises a first magnetic ring 73, six first magnetic columns 72projecting respectively from the first magnetic ring 73 downward, andfirst magnetic tips 71 which are formed by bending a lower end portionof each first magnetic column 72 inward so as to face the lower end faceof the magnet ring 63.

The second soft magnetic member 80 comprises a second magnetic ring 83,six second magnetic columns 82 projecting respectively from the secondmagnetic ring 83, and second magnetic tips 81 which are formed bybending an upper end portion of each second magnetic column 82 inward soas to face the lower end face of the magnet ring 63.

The first magnetic ring 73 and the second magnetic ring 83 are removedfrom each other in the direction of the rotation axis such that thefirst magnetic tips 71 and the second magnetic tips 81 are arrangedalternately at equal angular intervals on an identical plain which isorthogonal to the rotation axis of the torsion bar 51. The firstmagnetic ring 73 and the second magnetic ring 83 are formed to have apredetermined width in the radial direction. The first magnetic ring 73and the second magnetic ring 83 are formed into a continuous ring-shape.It is also possible to form the first magnetic ring 73 and the secondmagnetic ring 83 in a C-shaped ring having a slit.

Referring again to FIG. 4, the first magnetic tip 71 and the secondmagnetic tip 81 are disposed in a predetermined rotation position suchthat radial lines connecting the center of the torsion bar 51 and thecenter of the respective first magnetic tips 71 and radial linesconnecting the center of the torsion bar 51 and the center of therespective second magnetic tips 81 correspond to the border between theN-pole and the S-pole of each of the six arc-shaped magnets forming themagnet ring 63 when the power steering device is in a neutral positionin which no torque is exerted on the torsion bar 51.

Referring again to FIGS. 3A and 3B, the first magnetic columns 72 andthe second magnetic columns 82 are disposed in parallel with therotation axis of the torsion bar 51. Each of the first magnetic columns72 and the second magnetic columns 82 is formed into a flat-plate-shape.When assembled into the rotating magnetic circuit 69, the first magneticcolumns 72 and the second magnetic columns 82 form a planar shape of adodecagon about the center of the torsion bar 51.

It is also possible to form each of the first magnetic columns 72 andthe second magnetic columns 82 in a curved shape such that the firstmagnetic columns 72 and the second magnetic columns 82, when assembledinto the rotating magnetic circuit 69, form a cylindrical shape which iscoaxial with the torsion bar 51.

The first magnetic columns 72 extend downward from the first magneticring 73 and pass along the outer periphery of the magnet ring 63 toreach the first magnetic tips 71 located below the magnet ring 63. Apredetermined space is kept between the first magnetic columns 72 andthe magnet ring 63 to prevent a magnetic short-circuit therebetween. Thesecond magnetic columns 82 extend upward from the second magnetic ring83 to reach the second magnetic tips 81. The first magnetic columns 72and the second magnetic columns 82 therefore extend in oppositedirections to reach the first magnetic tips 71 and the second magnetictips 81, which are located in the same plane.

According to the above construction, the size of the rotating magneticcircuit 69 can be made short in the direction of the rotation axis whilepreserving enough space between the first magnetic ring 73 and thesecond magnetic ring 83 to prevent a magnetic short-circuittherebetween.

The first magnetic ring 73, the first magnetic columns 72, and the firstmagnetic tips 71 are manufactured in advance by performing press-workingon a soft magnetic plate material to form a one-piece first softmagnetic member 70. The second magnetic ring 83, the second magneticcolumns 82, and the second magnetic tips 81 are also manufactured inadvance by performing press-working on a soft magnetic plate material toform a one-piece second soft magnetic member 80. It is also possible tomanufacture these members by means of metal casting or sintering.

When the first soft magnetic member 70 and the second soft magneticmember 80 are manufactured by means of metal casting or sintering, it ispossible to form the first magnetic columns 72 and the second magneticcolumns 82 into a block shape that is thicker than the press-formedmagnetic columns in the radial direction with respect to the torsion bar51.

The first soft magnetic member 70 and the second soft magnetic member 80are preferably made as identical members having an identical shape andsize. By reversing the directions of assembling, the two identicalmembers serve as the first soft magnetic member 70 and the second softmagnetic member 80.

It is still possible to form the soft magnetic member 70 and the secondsoft magnetic member 80 in different shapes. For example, the first softmagnetic member 70 may be formed in a flat shape in which the firstmagnetic columns 72 are omitted and the first magnetic tips 71 projectinward directly from the first magnetic ring 73. In this case, only thesecond magnetic columns 82 of the second soft magnetic member 80function to provide a space between the first magnetic ring 73 and thesecond magnetic ring 83 in the direction of the rotation axis of thetorsion bar 51 so as to prevent a magnetic short-circuit between thefirst magnetic ring 73 and the second magnetic ring 83.

Similarly, the second soft magnetic member 80 may be formed in a flatshape in which the second magnetic columns 82 are omitted and the secondmagnetic tips 81 project inward directly from the second magnetic ring83. In this case, only the first magnetic columns 72 provided in thefirst soft magnetic member 70 function to provide a space between thefirst magnetic ring 73 and the second magnetic ring 83 in the directionof the rotation axis of the torsion bar 51 so as to prevent a magneticshort-circuit between the first magnetic ring 73 and the second magneticring 83.

The fitting member 77 is made from ferrous metal and formed in acylindrical shape. A resin-molded body 87 is used for integrating thefitting member 77, the first soft magnetic member 70, and the secondsoft magnetic member 80 into one-piece.

Referring again to FIG. 1, a plurality of holes 29 are formed on theouter periphery of the output shaft 20. The fitting member 77 has alower end 76 which covers the holes 29 in a state where the rotatingmagnetic circuit 69 has been press-fitted onto the output shaft 20. Oncethe rotating magnetic circuit 69 has been press-fitted onto the outputshaft 20, the fitting member 77 is secured onto the output shaft 20 soas not to displace in the axial and rotation directions by punching thecorresponding parts of the lower end into the holes 29 using a stakingjig.

By thus integrating the fitting member 77, the first soft magneticmember 70, and the second soft magnetic member 80 into one piece via theresin-molded body 87, no magnetic short-circuits occur among the fittingmember 77, the first soft magnetic member 70, and the second softmagnetic member 80 even if the fitting member 77 is constructed from aferrous metal.

It is possible to construct the fitting member 77 from a nonmagneticmaterial such as aluminum, but constructing the fitting member 77 from aferrous metal helps to reduce the manufacturing cost of the torquesensor 50.

The resin-molded body 87 is formed as follows. The fitting member 77,the first soft magnetic member 70, and the second soft magnetic member80 are first disposed in a predetermined die. A molten thermoplasticresin is then funneled into the die. The resin-molded body 87 isobtained by cooling the molten thermoplastic resin thus molded. Thematerial for the resin-molded body 87 is not limited to moltenthermoplastic resin, and thermosetting resin or reaction curing resinmay be used instead.

Referring again to FIGS. 3A and 3B, the first magnetic ring 73 has aplurality of positioning holes 74 formed at regular angular intervals.The second magnetic ring 83 has a plurality of positioning holes 84formed at regular angular intervals. When the resin-molded body 87 ismolded, the magnetic ring 73 and the second magnetic ring 83 arepositioned precisely in the die with respect to the rotation directionby inserting a positioning jig into the positioning holes 74 and 84.

An annular groove 78 is formed on the outer periphery of the fittingmember 77 in advance. The fitting member 77 is provided with a pluralityof rotation blocking holes 79 opening onto the annular groove 78. Whenforming the resin-molded body 87, the annular groove 78 and the rotationblocking holes 79 are filled up with the resin, thereby preventing thefitting member 77 from displacing in the axial and rotation directionswith respect to the resin-molded body 87.

Referring again to FIG. 2, the die is formed in advance in such a shapethat twelve inward depressed portions 88 are formed on the innerperiphery of the resin-molded body 8 to accommodate the first magneticcolumns 72 and the second magnetic columns 82.

By forming the die in this way, the resin funneled into the die to moldthe resin-molded body 87 is prevented by the die from contacting thefirst magnetic columns 72 and the second magnetic columns 82. Such anarrangement of the die is preferable in preventing an internal stressfrom being generated in the first magnetic columns 72 and the secondmagnetic columns 82 due to a forming pressure of the resin during theprocess of solidification and shrinking, or due to thermal expansion orthermal shrinkage of the resin after the resin-molded body 87 is molded.

Internal stress causes the magnetic flux transmitting performance of thefirst magnetic columns 72 and the second magnetic columns 82 todeteriorate. By forming the die to provide the depressed portions 88,the first magnetic columns 72 and the second magnetic columns 82 can bepositioned easily with respect to the die.

The die is formed such that the depressed portions 88 are provided onthe outside of the first magnetic columns 72 and the second magneticcolumns 82. It is however possible to form the die such that thedepressed portion 88 is formed on the inside of the first magneticcolumns 72 and the second magnetic columns 82.

The fixed magnetic circuit 90 comprises a first magnetic collecting ring91 facing the outer periphery of the first magnetic ring 73, a secondmagnetic collecting ring 92 facing the outer periphery of the secondmagnetic ring 83, a first magnetic collecting yoke 93 connected to thefirst magnetic collecting ring 91, and a second magnetic collecting yoke94 connected to the second magnetic collecting ring 92.

The first magnetic collecting ring 91 is constructed from a softmagnetic material into a C-shaped ring which has a slit 91A. The secondmagnetic collecting ring 92 is constructed from a soft magnetic materialinto a C-shaped ring which has a slit 92A. The first magnetic collectingring 91 and the second magnetic collecting ring 92 are formed in anidentical shape. The first magnetic collecting ring 91 and the secondmagnetic collecting ring 92 are fixed on the inner periphery of thehousing 30.

Referring again to FIG. 1, a first annular groove 31 into which thefirst magnetic collecting ring 91 is fitted and a second annular groove32 into which the first annular groove 31 is fitted are formed on theinner periphery of the housing 30. The first magnetic collecting ring 91narrows the width of the slit 91A when it is fitted into the firstannular groove 31. The second magnetic collecting ring 92 narrows thewidth of the slit 92A when it is fitted into the second annular groove32. When the widths of the slits 91A and 92A are narrowed, the effect ofthe magnetic gap formed by the slits 91A and 92A also decreases. Thehousing 30 is formed from a nonmagnetic material such as aluminum.

The depth of the first annular groove 31 is set to be smaller than thethickness of the first magnetic collecting ring 91 in the radialdirection. The depth of the second annular groove 32 is set to besmaller than the thickness of the first magnetic collecting ring 92 inthe radial direction. As a result, the first magnetic collecting ring 91projects inward from an inner peripheral surface 38 of the housing 30 ina state where it is fitted into the first annular groove 31. The secondmagnetic collecting ring 92 projects inward from the inner peripheralsurface 38 of the housing 30 in a state where it is fitted into thesecond annular groove 32.

The first magnetic collecting ring 91 has a central portion with respectto the axial direction which faces the outer periphery of the firstmagnetic ring 73 at a predetermined gap. The second magnetic collectingring 92 has a central portion with respect to the axial direction whichfaces the outer periphery of the second magnetic ring 83.

When the fixed magnetic circuit 90 is fitted into the housing 30,staking parts 33 are formed on the inner periphery of the housing 30along the first annular groove 31 by punching the inner peripheralsurface 38 of the housing 30 using a jig to deform the portions on bothsides of the first annular groove 31, thereby securing the firstmagnetic collecting ring 91 within the first annular groove 31.

Similarly, staking parts 34 are formed on the inner periphery of thehousing 30 along the second annular groove 32 by punching on the innerperipheral surface 38 of the housing 30 using a jig to deform theportions on both sides of the second annular groove 32, thereby securingthe second magnetic collecting ring 92 within the second annular groove32.

According to this process, looseness is eliminated in the first magneticcollecting ring 91 and the second magnetic collecting ring 92 and therotation of the first magnetic collecting ring 91 and the secondmagnetic collecting ring 92 with respect to the housing 30 is prevented.

It is possible to form the housing 30 from a resin. In this case, thefirst magnetic collecting ring 91 and the second magnetic collectingring 92 may be fixed to the housing 30 by heat staking.

The first magnetic collecting yoke 93 and the second magnetic collectingyoke 94 are fixed in advance to the interior of a sensor housing 39. Bypress-fitting the sensor housing 39 into a lateral opening of thehousing 30, the first magnetic collecting yoke 93 comes into contactwith a back portion of the first magnetic collecting ring 91 on theopposite side of the slit 91A and the second magnetic collecting yoke 94comes into contact with a back portion of the second magnetic collectingring 92 on the opposite side of the slit 92A.

Referring again to FIG. 2, the first magnetic collecting yoke 93 isformed into a block shape and has a pair of magnetic collectingprojections 93 a projecting downward. The second magnetic collectingyoke 94 is formed into a block shape and has a pair of magneticcollecting projections 94 a projecting upward. The pair of magneticcollecting projections 93 a and the pair of magnetic collectingprojections 94 a have end faces facing each other on both sides of apair of magnetic gaps 96.

Referring again to FIG. 1, a magnetic sensor 98 using a hall element isinserted into the pair of magnetic gaps 96. The magnetic sensor 98outputs signals representing a magnitude and direction of a magneticfield formed in the magnetic gaps 96 via a signal cable 97. The magneticsensor 98 may further comprise an amplifying circuit which amplifies thesignals generated by the hall element, a circuit for temperaturecompensation, or a circuit for noise filtering.

When the power steering device is in the neutral position in which notorque is exerted on the torsion bar 51, the first magnetic tips 71 ofthe first soft magnetic member 70 and the second magnetic tips 81 of thesecond soft magnetic member 80 face the N-poles and the S-poles of themagnet ring 63 evenly, thereby causing a magnetic short circuit betweenan N-pole and an adjacent S-pole. In this state, the magnetic flux ofthe magnet ring 63 is not transmitted to the rotating magnetic circuit69 and the fixed magnetic circuit 90.

When a driver of a vehicle operates the steering wheel, a torque in onedirection is input into the torsion bar 51 and the torsion bar 51undergoes torsional deformation according to the direction of the inputtorque.

Providing that the rotating magnetic circuit 69 has rotated clockwise inFIG. 4 with respect to the magnet ring 63 as a result of the torsionaldeformation of the torsion bar 51, the total area of the first magnetictips 71 facing the N-poles increases and the total area of the secondmagnetic tips 81 facing the S-poles increases.

As a result, the magnetic flux generated by the magnet ring 63 istransmitted to the field magnetic circuit 90 via the rotating magneticcircuit 69, and the magnetic sensor 98 outputs signals in response tothe magnitude and the direction of the magnetic field formed in themagnetic gaps 96.

The magnetic path formed through the rotating magnetic circuit 69 andthe fixed magnetic circuit 90 in this state starts from the N-poles ofthe magnet ring 63, and then passes through the first magnetic tips 71,the first magnetic columns 72, the first magnetic ring 73, the firstmagnetic collecting ring 91, the first magnetic collecting yoke 93, thesecond magnetic collecting yoke 94, the second magnetic collecting ring92, the second magnetic ring 83, the second magnetic columns 82, and thesecond magnetic tips 81 to reach the S-poles of the magnet ring 63.

The torsion bar 51 undergoes a torsional deformation in response to theinput torque. As the difference in the N-pole facing area and the S-polefacing area of the first magnetic tips 71 and the difference in theS-pole facing area and the N-pole facing area of the second magnetictips 81 increases, the magnitude of the magnetic field formed in themagnetic gap 96 increases and the output signals from the magneticsensor 98 vary accordingly.

It should be noted that the number of the poles of the magnet ring 63can be set arbitrarily as long as it is equal to or greater than two.Providing that the area of the first soft magnetic member 70 facing themagnet ring 63 is identical to the area of the second soft magneticmember 80 facing the magnet ring 63, the magnetic flux densitytransmitted to the magnetic sensor 98 is increased by increasing thenumber of poles of the magnet ring 63.

The first magnetic ring 73 and the second magnetic ring 83 must beremoved from each other in the direction of the rotation axis so asprevent a magnetic short-circuit therebetween.

In the torque sensor according to the aforesaid prior art, both thefirst magnetic ring and the second magnetic ring are located under themagnet and both the first magnetic columns and the second magneticcolumns are formed upward toward the first magnetic tips and the secondmagnetic tips, respectively. As a result, the first magnetic columns canbe made short, but the second magnetic columns inevitably become long.Therefore it is difficult to manufacture the second soft magnetic membercomprising the second magnetic columns by means of press-working.

In the torque sensor 50 according to this invention, the magnet ring 63is disposed between the first magnetic ring 73 and the second magneticring 83 with respect to the direction of the rotation axis and the firstmagnetic columns 72 and the second magnetic columns 82 are arranged toproject in opposite directions.

More specifically, the first magnetic column 72 projects downward fromthe first magnetic ring 73, thereby passing the outside of the magneticforce generating part 60 in the direction of the rotation axis to reachthe first magnetic tips 71. The second magnetic columns 82 projectupward from the second magnetic ring 83 so as to approach the magneticforce generating part 60 in the direction of the rotation axis to reachthe second magnetic tips 81.

According to the above construction of the rotating magnetic circuit 69,the length of the first magnetic columns 72 and the length of the secondmagnetic columns 82 in the direction of the rotation axis can be madeshorter with respect to the distance between the first magnetic ring 73and the second magnetic ring 83. By shortening the length of the firstmagnetic columns 72 and the length of the second magnetic columns 82 inthis way, the first soft magnetic member 70 and the second soft magneticmember 80 can be manufactured by means of press-working.

Since the press-working requires less material than metal casting orsintering, manufacturing the first soft magnetic member 70 and thesecond soft magnetic member 80 by means of press-working lowers themanufacturing cost of the torque sensor 50.

Since press-working manufactures a member which is thinner than a membermanufactured by metal casting or sintering, the cross-sectional area ofthe magnetic path which the member can provide is inevitably smaller.However, by constructing the first soft magnetic member 70 and thesecond soft magnetic member 80 using a high-density material, magnetichysteresis characteristics of the torque sensor 50 can be improved.

Further, since the first magnetic columns 72 are disposed to surroundthe magnet ring 63, the space required in the direction of the rotationaxis for disposing the first soft magnetic member 70 becomes small,thereby realizing a compact and lightweight torque sensor.

The contents of Tokugan 2008-093636, with a filing date of Mar. 31, 2008in Japan, are hereby incorporated by reference.

Although the invention has been described above with reference tocertain embodiments, the invention is not limited to the embodimentsdescribed above. Modifications and variations of the embodimentsdescribed above will occur to those skilled in the art, within the scopeof the claims.

For example, application of the torque sensor according to thisinvention is not limited to a power steering device for a vehicle. It iswidely applicable for the detection of a torque acting betweenrelatively rotating objects connected by a torsion bar.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A torque sensor comprising: a housing; a torsion bar inserted intothe housing, the torsion bar having a rotation axis, the torsion barhaving opposite first and second ends; a magnetic force generating partwhich rotates together with the first end of the torsion bar whilegenerating a magnetic flux in a direction of the rotation axis; arotating magnetic circuit which rotates together with the second end ofthe torsion bar, the rotating magnetic circuit comprising a first softmagnetic member and a second soft magnetic member, the first softmagnetic member comprising a first magnetic ring disposed around thetorsion bar, a first magnetic tip disposed to face the magnetic forcegenerating part in the direction of the rotation axis, and a firstmagnetic column connecting the first magnetic tip magnetically to thefirst magnetic ring, and the second soft magnetic member comprising asecond magnetic ring disposed around the torsion bar in a positionoffset from a position of the first magnetic ring in the direction ofthe rotation axis, a second magnetic tip disposed such that the secondmagnetic tip and the first magnetic tip are disposed on an identicalplane orthogonal to the rotation axis, and a second magnetic columnconnecting the second magnetic tip magnetically to the second magneticring, the second magnetic column and the first magnetic column beingoriented oppositely; a fixed magnetic circuit fixed to the housing tosurround the rotating magnetic circuit so as to accept without contact amagnetic flux from the rotating magnetic circuit; and a magnetic sensorwhich detects a magnetic flux density in the fixed magnetic circuit. 2.The torque sensor as defined in claim 1, wherein the magnetic forcegenerating part comprises a magnet ring which is disposed coaxially withthe torsion bar on the outside thereof and has an N-pole and an S-poleformed alternately in a peripheral direction at equal angular intervals,and the first magnetic tip has an N-pole facing area and an S-polefacing area that are equal to each other, while the second magnetic tiphas an N-pole facing area and an S-pole facing area that are equal toeach other, in a neutral position where the torsion bar does not undergotorsional deformation.
 3. The torque sensor as defined in claim 1,wherein the first magnetic ring and the first magnetic tip are offsetfrom the magnetic force generating part in different directions alongthe rotation axis, the first magnetic column is arranged to pass theoutside of the magnetic force generating part so as to connect the firstmagnetic ring and the first magnetic tip.
 4. The torque sensor asdefined in claim 3, wherein the first soft magnetic member and thesecond soft magnetic member are formed into an identical shape anddisposed in opposite directions with respect to the direction of therotation axis.
 5. The torque sensor as defined in claim 1, wherein thefirst soft magnetic member and the second soft magnetic member areformed by press-working a soft magnetic plate material.
 6. The torquesensor as defined in claim 1, wherein the rotating magnetic circuitfurther comprises a resin-molded body which fixes the relative rotationposition between the first soft magnetic member and the second softmagnetic member, and a fixing member which fixes the first soft magneticmember and the second soft magnetic member to the second end of thetorsion bar via the resin-molded body.
 7. The torque sensor as definedin claim 6, wherein the resin-molded body comprises depressed portionswhich accommodate the first magnetic column and the second magneticcolumn.
 8. The torque sensor as defined in claim 7, wherein a minute gapis formed between the depressed portion and the first magnetic column aswell as between the depressed portion and the second magnetic column. 9.The torque sensor as defined in claim 1, wherein the fixed magneticcircuit comprises a first magnetic collecting ring which surrounds thefirst magnetic ring and a second magnetic collecting ring whichsurrounds the second magnetic ring, a first magnetic collecting yokecontacting the first magnetic collecting ring, and a second magneticcollecting yoke contacting the second magnetic collecting ring, and themagnetic sensor is interposed between the first magnetic collecting yokeand the second magnetic collecting yoke.